1. Field of the Invention
This invention relates to FLADE aircraft gas turbine engine and more particularly to construction and operating method of such an engine to reduce inlet spillage drag.
2. Description of Related Art
A considerable effort has been made toward developing high performance variable cycle gas turbine engines. These types of engines have a unique ability to operate efficiently at various thrust settings and flight speeds both subsonic and supersonic. An important feature of the variable cycle gas turbine engine which contributes to its high performance is its capability of maintaining a substantially constant inlet airflow as its thrust is varied. This feature leads to important performance advantages under less than full power engine settings or maximum thrust conditions, such as during subsonic cruise. The effect of maintaining inlet airflow as thrust is reduced is to decrease such performance penalties as inlet spillage drag and afterbody closure drag, both of which have a considerable detrimental effect on the overall efficiency, size, weight, and performance of the aircraft and its engine.
Certain variable cycle engines, such as those described in U.S. Pat. Nos. 4,068,471 and 4,285,194, assigned to the same assignee as the present invention, achieve relatively constant airflow as thrust is varied by changing the amount of fan bypass flow with a valve system referred to as a variable area bypass injector (VABI). As engine thrust is decreased, the VABI increases bypass flow to offset decreasing core engine flow resulting in a relatively constant total engine flow, thus exhibiting the performance benefits described above. However, it uses very expensive multi-stage fan air to do so. Furthermore, it is somewhat limited in its ability to ingest the air at subsonic part power engine settings and engine operation because it has to be sized to match the flow conditions in the bypass duct into which is dumped some very highly pressurized air. This fan air is divided between the core flow and the bypass flow and therefore is controlled by the various bypass flow control mechanisms such as the VABI's. This in turn limits the degree to which the bypass air can be used to avoid spillage. Typically, conventional variable cycle engines have a limited range of thrust settings for a given Mach No. (particularly subsonic levels e.g. Mach No.=0.8-0.9) through which essentially constant airflow can be maintained and specific fuel consumption can be minimized. Therefore it is desirable to construct and operate an aircraft gas turbine engine able to maintain inlet airflow at subsonic part power thrust settings more efficiently and over a broader flight envelope than is available in the prior art.
One particular type of variable cycle engine called a FLADE engine (FLADE being an acronym for "fan on blade") is characterized by an outer fan driven by a radially inner fan and discharging its flade air into an outer fan duct which is generally coannular with and circumscribes an inner fan duct circumscribing the inner fan. One such engine, disclosed in U.S. Pat. No. 4,043,121, entitled "Two Spool Variable Cycle Engine", by Thomas et al., provides a flade fan and outer fan duct within which variable guide vanes control the cycle variability by controlling the amount of air passing through the flade outer fan duct.
There remains an important need to provide a high performance aircraft gas turbine engine, particularly of the variable cycle type, that is capable of maintaining an essentially constant inlet airflow over a relatively wide range of thrust at a given set of subsonic flight ambient conditions such as altitude and flight Mach No. in order to avoid spillage drag and to do so over a range of flight condition. This capability is particularly needed for subsonic part power engine operating conditions.